The invention relates generally to network communications. More specifically, the invention relates to systems and methods that provide a distributed restoration signaling protocol for mesh optical networks, including opaque, transparent, and translucent Optical cross-Connects (OXCs).
An OXC is used to switch high-speed optical signals in a fiber optic network. There are several implementations of OXCs. One is implemented in the electronic domain. All the input optical signals are converted into electronic signals using optical transponders after they are demultiplexed. The electronic signals are then switched by an electronic switch module. Finally, the switched electronic signals are converted back into optical signals using optical transponders again, and then the resulting optical signals are multiplexed by optical multiplexers onto output fibers. This is known as Optical-Electrical-Optical (OEO) switching.
OXCs based on OEO switching limit the maximum bandwidth of the signal due to their electronic circuits. OEO switching is also expensive due to the number of optical transponders. One advantage is that the optical signals are regenerated, so they leave the OXC free of dispersion and attenuation. An electronic OXC is also called an opaque OXC.
Switching optical signals in an all-optical device is the second implementation. Such a switch is called a transparent OXC. Optical signals are demultiplexed, and then the demultiplexed wavelengths are switched by optical switch modules. After switching, the optical signals are multiplexed onto output fibers by optical multiplexers. This switch architecture keeps the data rate and data contents transparent. Because the signals are kept in the optical format, the transparent OXC architecture does not allow easy optical signal quality monitoring.
In an opaque optical network, an optical signal service connection is terminated at each hop along its path and is regenerated for the next hop. A connection is an end-to-end signal path where one end is an ingress OXC and the other end is an egress OXC. Opaque optical networks do not have wavelength continuity constraint issues and each OXC provides wavelength conversion automatically, allowing conversion from one wavelength to another. However, opaque OXCs and interfaces are more expensive.
In contrast, in a transparent optical network, an optical signal service connection is not terminated at each hop along its path, meaning that each link OXC does not provide wavelength conversion. The wavelengths assigned to a service connection must remain the same throughout the whole path providing wavelength continuity. Transparent optical networks have lower cost interfaces, floor space and power requirements as compared with opaque optical networks.
Due to the physical nature of optical fiber, an optical signal has to be regenerated after a predetermined fiber distance to prevent signal degradation that cannot be restored. This distance limit is called “optical-reach.” An optical signal is able to travel a relatively long distance without requiring OEO regeneration, such that the signal is able to travel multiple hops throughout the optical network. A regenerator is required when a service connection length equals its optical-reach. Signal regeneration may also be used in the service connection if a different wavelength must be assigned even if the path length is within its optical-reach. Since regenerators are expensive, the number of regenerators are optimized (minimized) when provisioning an optical service connection and its restoration.
A compromise between opaque and transparent OXCs is a translucent OXC. Translucent OXCs include a switch stage which comprises an optical switch module and an electronic switch module. Optical signals passing through the switch stage can be switched either by the optical switch module or the electronic switch module. In most cases, the optical switch module is preferred for the purpose of transparency. When the optical switch module's switching interfaces are all busy or an optical signal needs signal regeneration through OEO conversion, the electronic module is used. Translucent OXCs provide a compromise of full optical signal transparency and comprehensive optical signal monitoring. It also provides the possibility of signal regeneration at each OXC. Embodiments focus on transparent OXCs with regenerators, which is a special case of a translucent OXC.
Ultra Long Haul (ULH) technologies for Dense Wavelength Division Multiplexing (DWDM) transports are being deployed due to their high-capacity and capital savings. A first generation ULH network typically includes a set of point-to-point linear systems with each linear system having two terminals. Between the two terminals, there may be one or more Reconfigurable Optical Add-Drop Multiplexers (ROADMs) where traffic may be added or dropped, or expressed through optically by degree-2 ROADMs.
Using these ULH systems, a wavelength connection is able to travel a long distance such as 1,500 km or more without requiring OEO regeneration. A connection using the same wavelength without OEO regeneration is referred as a lightpath. An OEO regenerator is needed when a lightpath length equals its optical-reach. When a connection has to travel through two linear ULH systems, an OEO regenerator is also needed even if its length is within its optical-reach. Since the OEO regenerators are expensive devices, first generation ULH networks were not inexpensive. They also complicate dynamic reconfiguration and restoration in the optical network.
To reduce the cost of OEO regeneration and enable automatic reconfigurability and dynamic restoration via wavelength switching and tuning, next generation optical networks are moving toward an all-optical mesh network. These configurations convert the terminals and degree-2 ROADMs to higher-degree ROADMs to switch and route wavelengths optically, which are known as Photonic cross-Connects (PXCs).
Both IP and wavelength services have stringent quality requirements for their high priority traffic. One requirement is resiliency against network failures that include performing a sub-second restoration for high priority traffic that has experienced a single failure, and for a small percentage of mission critical traffic, the ability to restore after experiencing multiple failures.
Unlike opaque optical networks, where OEO conversion occurs in the signal path at all OXCs, a transparent ULH network OXC does not have OEO conversion/regeneration functions and the signal path may require OEO regenerators for some OXCs.
Shared mesh restoration has been used in opaque optical networks where all restoration channels on each link are pre-reserved and each OXC has wavelength conversion capability. When a failure is detected, a pre-planned restoration path will be dynamically established by cross-connecting any available channel on each link along the pre-planned restoration path. U.S. Pat. No. 6,970,417 B1 (assigned to the assignee of the present invention) discloses shared mesh restoration for opaque optical networks wherein the restoration methods and systems include error detection at the add/drop ports of network nodes. However, in a transparent optical network, the OXC itself does not have wavelength conversion capability and opaque optical network shared mesh restoration schemes cannot be applied to transparent optical networks directly. Therefore, shared mesh restoration with standbys restoration scheme may be used in transparent optical networks. Here a standby means one or more pre-configured lightpaths with the same optical path and different wavelengths for network restoration. Then a restoration path can be configured through a series of restoration standbys via regenerators.
In shared mesh restoration with standbys, each OXC includes a pool of OEO regenerators. However, they are not dedicated to a specific restoration path prior to a failure. Instead, depending on the failure, they dynamically are tuned and pointed appropriately so that they are able to establish a restoration path. In this case, the regenerators are shared among multiple restoration paths.
The challenge in reliable optical network design is to provide fast restoration in conjunction with restoration capacity in a cost effective manner. A robust and fast distributed restoration signaling protocol for mesh transparent optical networks is needed.